California Democrats completed their sweep of the congressional delegation in Orange County on Saturday as Gil Cisneros defeated Young Kim, a Republican, to capture a fourth seat in what had once been one of the most conservative Republican bastions in the nation.
The victory by Mr. Cisneros, a philanthropist, was declared by The Associated Press. It completes what has amounted to a Democratic rout in California this year. Democrats set out to capture seven Republican-held seats where Hillary Clinton defeated President Trump in 2016, including four in Orange County. They won six of them.
Representative David Valadao, from the Central Valley, is the only Republican who survived the Democratic onslaught in those seven districts, according to The Associated Press. His margin has shrunk as mail-in votes have continued to be counted. The deadline for counting those votes in California is Dec. 7.
With Mr. Cisneros’s victory, Democrats now control all four House seats in Orange County — the birthplace of Richard M. Nixon and modern-day conservatism. The party also won supermajorities in the California Assembly and Senate, while the party’s candidate for governor — Gavin Newsom, the lieutenant governor — easily turned back a Republican challenge. Democrats control every statewide elected position in California.
Before this election, the 53-member California congressional delegation included 39 Democrats and 14 Republicans. Assuming Mr. Valadao keeps his lead, after this year’s midterms it will be 45 Democrats and eight Republicans.
Mr. Cisneros and Ms. Kim were competing for the seat left open after Representative Ed Royce, who has represented the area since 1993, decided not to seek re-election. Mr. Cisneros won by about 3,500 votes, receiving 50.8 percent of the votes cast.
Mr. Cisneros is a former Navy officer who became a millionaire after winning the California state lottery in 2010. He and his wife turned to philanthropy after that. He is a former Republican who left the party in 2008 to become a Democrat.
Another Republican, Representative Darrell Issa, who represented San Diego and Orange Counties, also decided not to seek re-election in what clearly was a challenging political environment for Republicans, given Mr. Trump’s unpopularity and demographic shifts in Southern California.
Democrats easily captured Mr. Issa’s seat as Mike Levin, an environmental lawyer, defeated Diane Harkey, a Republican and former member of the Assembly.
The changing political tides in Orange County were captured in the ethnic dynamics of the race to succeed Mr. Royce. Ms. Kim was seeking to become the first Korean-American woman in Congress, but faced a challenge from Mr. Cisneros, a Latino in a state where Latino voters have become an increasingly powerful force.
Aides to Mr. Cisneros said they were aggressively seeking support from Asian-American voters — they dispatched campaign workers who spoke Korean and Mandarin.
The results were a setback for the House majority leader, Kevin McCarthy, a Bakersfield Republican who had invested energy and money into trying to salvage his delegation. Mr. McCarthy, who will be the minority leader in the next Congress, after Democrats seized control of the House, helped champion a ballot initiative that would overturn a gasoline tax passed by the Legislature last year to pay for road repair. The initiative was intended to bring Republicans out to vote in these endangered Republican districts.
The initiative was defeated, and political analysts said the results suggested that any increase in Republican turnout caused by the gas tax was overwhelmed by turnout among Democratic voters. The Democratic Congressional Campaign Committee had seized on California as a leading front of its battle to take back Congress, opening an office there in 2017, and flooding the district with money and workers.
The idea of making such an effort in Orange County — and in districts with a Republican registration edge — would have seemed unthinkable not long ago. But the area has been becoming Democratic in recent years, as it has become younger and more ethnically diverse.
Mrs. Clinton defeated Mr. Trump by more than four million votes in California and won in Orange County. Republicans said the president’s continued attacks on California since the election — along with his advocating tough immigration measures in the final weeks of the campaign — had created obstacles for Republican candidates already in a tough political environment.
California Republicans have seen their enrollment in steady decline over the past 20 years. Some party members — led by Arnold Schwarzenegger, the former governor — have sought to push the party to moderate its positions on issues like immigration, in an effort to appeal to more voters.
But those efforts have largely been rebuffed; many of the Republican candidates for Congress in California this year — in particular, Representative Dana Rohrabacher and Representative Mimi Walters — stood by Mr. Trump through the election process. Both of them lost their seats to Democratic challengers.